Features to 3+ planets gravity assist optimization [Enhancement]

[Kerbinator-Fras]

Well I think it's awesome to use principia,but I always love to push things all the way to limit.Recently I'm encountering some problems when deeply optimising E-V-V-M-M-M-M MESSENGER-alike trajectories starting at 2029.8.3 , 24 years after MESSENGER launch window. 1.The ability to add special points of other frame onto another frame Example:showing MCSA Mercury Periapsis and Mercury-Sun plane AN/DN on the SCI at exactly the same time(for example show Mercury-Sun AN at T+1209d,and ti will be displayed on 1209d position at SCI.This make the AN/DN show on inertial frame which benefit gravity assist plot a lot.Showing Mercury periapsis on SCI is also good since it let you know about phase angle problems. 2.different colors between PRmanoeurves(excluding in-burn lines as light pink) I know from some test that the steps is applied between trajectories,so show different part of trajectories in different colors(can be customized,showing on the property of each manoeurve That's to focus on one part of the trajectory,for example,when optimising V-M transfer,I'm confused by these E-V-V lines in the MCSA frame and it disturbed me from finding the optimization lines for V-M transfer. 3.add revert to old timeline Well in these latter version of PR,the t initial is based on the last manoeurve.So when I want to set up node at some place,for instance a specific periapsis,since the periapsis time is calculated from the start of plan,I have to calculate t initial.Well you can remain to use the current t initial form,but display that on manoeurve properties.